The low-pressure mercury vapour discharge light sources made with a luminescent coating, particularly the fluorescent lamps show the drawback that their luminous flux generally decreases with increasing work time. The intensity of this process depends on the kind of the light source and especially on the type of the luminescent material applied. Lots of methods were reported for diminishing the intensity, and particularly for avoiding this process. The methods forming the art are, however, different because the processes themselves differ from one another, as it is described e.g. in the introductory part of the GB-PS 1,343,250.
One of the most important factors influencing the intensity of the process mentioned is that the luminescent material is electrically not fully neutral. This follows from the fact that activating ions should be introduced into the crystalline lattice of the crystalline substance being the basis of the luminescent material. The presence of the activating ions is necessary for ensuring radiation of light characterized with the required wavelength. The excitation of the luminescent material is generally caused by the ultraviolet radiation of the mercury vapour taking part in the discharge process.
The activating ions can be introduced into the crystalline lattice in a relatively sophisticated manner which generally requires the application of anions in a surplus for ensuring the stability of the luminescent material. The surplus is related to the stoichiometric amounts and it results in the negative charge of the stabilized luminescent material. The luminescent material having negative charge attracts the mercury ions having positive charge and being present in the discharge space. The mercury ions are absorbed on the surface of the luminescent material and cause turning its colour to grey. In another process the luminescent material receives the mercury ions and the ions make the cations of higher electronegativity leave the material as described in the specification of the Hungarian Patent No. 181,471 granted to N. V. Philips Gloeilampenfabrieken, Eindhoven (Holland), on the basis of an application derived from the application NL-77.09263 filed on Aug. 23, 1977. The disadvantage of this process is that the luminous flux emitted by the light source is slowly decreasing, the light source can not ensure the light efficiency following from the physical features of the luminescent material when applying the prescribed other, particularly electric parameters. This process is shown among others in the GB-PS 1,229,038 with reference to the silicate based luminescent materials wherein the specification mentions that for diminishing the extent of the process of absorbing mercury it is advantageous to decrease the surplus of the silicon dioxide. Therefore zinc borate of specific composition including a surplus of zinc oxide is mixed with the luminescent material of known composition. The blend should undergo a heat treatment for assuring its desired structure and composition. The method is relatively sophisticated and requires parameters to be controlled with high accuracy. A further drawback is linked with the presence of zinc which is disadvantageous as analysed in the HU-PS 181,471.
An improvement of the features of the calcium halophosphate luminescent material and especially slowing down the decrease of the luminous flux was proposed according to the specification of U.S. Pat. No. 3,887,725 disclosing the step of preparing a surface layer on the grains of the luminescent material by preparing a blend with the luminescent material having known composition and zinc orthophosphate or zinc borate forming a coating component. In this method the presence of zinc is disadvantageous as it was analysed in the HU-PS 181,471 cited above. The zinc component deteriorates the stability of the luminescent material against the high energy mercury ions of positive charge. Therefore it is desired to avoid the use of luminescent materials including zinc in the fluorescent tubes of small dimensions.
Another useful proposal became known from the GB-PS 1,191,974 wherein a method for "in situ" preparing a phosphate layer on the surface of the calcium halophosphate grains is shown, the method being based on a chemical process. Another chemical process resulting in similar phosphate coating can be found in the GB-PS 1,343,250 with the difference that the grains consist of a silicate type luminescent material.
The methods of preparing a coating give acceptable results. There is, however, no change with the respect that the negative charge of the surface of the luminescent material remains, and the problems linked with this fact can not be avoided in this way.